Hello, I am new to stan and I could not get the following simple measurement error model to sample.

I am using rstan_2.19.2, r version 3.6.1, under macOS Catalina 10.15.1

```
model {
data {
int <lower=0> N; \\ number of obs
vector[N] E; \\ obs error
vector[N] I_obs; \\ observations
}
parameters {
vector[N] I_true; \\ unknown true value
real I_tmu; \\ prior location
real<lower=0> sigma_t; \\ prior scale
}
model {
sigma_t ~ exponential(1);
I_tmu ~ normal(0,3);
I_true[N] ~ normal(I_tmu, sigma_t); \\ prior
I_obs ~ normal(I_true, E); \\ measurement model
}
"
```

the error message was

```
model{
starting worker pid=10312 on localhost:11250 at 17:55:58.824
starting worker pid=10326 on localhost:11250 at 17:55:59.195
starting worker pid=10340 on localhost:11250 at 17:55:59.549
starting worker pid=10354 on localhost:11250 at 17:55:59.932
SAMPLING FOR MODEL '68b329da9893e34099c7d8ad5cb9c940' NOW (CHAIN 1).
[1] "Error in sampler$call_sampler(args_list[[i]]) : "
[2] " Must use algorithm=\"Fixed_param\" for model that has no parameters."
error occurred during calling the sampler; sampling not done
SAMPLING FOR MODEL '68b329da9893e34099c7d8ad5cb9c940' NOW (CHAIN 2).
[1] "Error in sampler$call_sampler(args_list[[i]]) : "
[2] " Must use algorithm=\"Fixed_param\" for model that has no parameters."
error occurred during calling the sampler; sampling not done
SAMPLING FOR MODEL '68b329da9893e34099c7d8ad5cb9c940' NOW (CHAIN 3).
[1] "Error in sampler$call_sampler(args_list[[i]]) : "
[2] " Must use algorithm=\"Fixed_param\" for model that has no parameters."
error occurred during calling the sampler; sampling not done
SAMPLING FOR MODEL '68b329da9893e34099c7d8ad5cb9c940' NOW (CHAIN 4).
[1] "Error in sampler$call_sampler(args_list[[i]]) : "
[2] " Must use algorithm=\"Fixed_param\" for model that has no parameters."
error occurred during calling the sampler; sampling not done
some chains had errors; consider specifying chains = 1 to debughere are whatever error messages were returned
[[1]]
Stan model '68b329da9893e34099c7d8ad5cb9c940' does not contain samples.
[[2]]
Stan model '68b329da9893e34099c7d8ad5cb9c940' does not contain samples.
[[3]]
Stan model '68b329da9893e34099c7d8ad5cb9c940' does not contain samples.
[[4]]
Stan model '68b329da9893e34099c7d8ad5cb9c940' does not contain samples.
```

I really appreciate any direction